1. Field of the Invention
The present invention relates to a relay network system. More particularly, the present invention relates to an apparatus and method for transmitting/receiving a Channel State Information-Reference Symbol (CSI-RS) in a relay network system.
2. Description of the Related Art
For 4th-Generation (4G) mobile communication systems, technologies are being proposed for maximizing the efficiency of data transmission and for increasing the capacity of a system while guaranteeing the improvement of a data rate. 4G-based standards, such as Long Term Evolution-Advanced (LTE-A) or Institute of Electrical and Electronics Engineers (IEEE) 802.16m that is an International Mobile Telecommunications (IMT)—advanced candidate technology, include a relay technique as a scheme for improving a poor wireless channel environment caused by a shadow area and expanding a cell coverage at a small expense.
Recently, an agreement for the introduction of a Relay Node (RN) equipped with all functions of an enhanced Node B (eNB) has been reached. Discussions on a backhaul link structure and design between the eNB and the RN are presently being made. Because the RN performs communication with the eNB and a User Equipment (UE) in the same frequency band through a backhaul link and an access link, the RN may bring about serious performance degradation caused by self-interference when simultaneously supporting the backhaul link and the access link under circumstances where no isolation between a receiver and a transmitter is guaranteed.
FIG. 1 is a diagram illustrating a subframe supporting a backhaul link in a communication system supporting a Relay Node (RN) according to the conventional art.
Referring to FIG. 1, the RN time-division-multiplexes the backhaul link and the access link subframe units. An LTE-A system includes a scheme in which, as illustrated in FIG. 1, though an eNB transmits a signal to the RN, the RN sets a fake Multicast Broadcast Single Frequency Network (MBSFN) subframe 101 and transmits nothing to UEs.
The LTE-A standard is expediting the design of a new DL reference symbol such as De-Modulation-Reference Symbol (DM-RS)/Channel State Information-Reference Symbol (CSI-RS). The CSI-RS is for channel state measurement, and the design of inter-cell CSI-RS in the current standard is under discussion.
An inter-cell CSI-RS transmission scheme is mainly classified into three schemes:
Transmitting CSI-RSs of different cells in different subframes;
Transmitting CSI-RSs of different cells in different Resource Blocks (RBs); and
Transmitting CSI-RSs of different cells, for orthogonality, at the same subframe using Code Division Multiplexing (CDM), Time Division Multiplexing (TDM), or Frequency Division Multiplexing (FDM).
Among the three schemes, the scheme of transmitting the CSI-RSs at the same subframe is most effective when considering a Time Division Duplex (TDD) mode of the limited number of DownLink (DL) subframes. However, in a system time-division-multiplexing the backhaul link and the access link in subframe units as illustrated in FIG. 1, different RNs should simultaneously monitor a common CSI-RS subframe of an eNB, and the RN cannot simultaneously perform transmission and reception. Therefore, the use of the scheme of transmitting the CSI-RSs at the same subframe causes a problem illustrated in FIG. 2.
FIG. 2 illustrates a case of transmitting a CSI-RS in a communication system supporting an RN according to the conventional art.
Referring to FIG. 2, it is assumed that a period of transmission of a CSI-RS is 10 ms, and an eNB transmits the CSI-RS at a SubFrame#2 (SF#2) 201. As illustrated in FIG. 2, when the eNB transmits the CSI-RS to an RN at the SF#2 201, the RN cannot transmit a signal including the CSI-RS to a UE at a DL SF#2 203 because RNs should simultaneously monitor the SF#2 201 sent from the eNB. The system including the RN is thus not able to apply the scheme of transmitting CSI-RSs of different cells at the same subframe.
The system may transmit a CSI-RS of an eNB and a CSI-RS of an RN at different subframes but, because the RN cannot simultaneously perform transmission and reception, when the eNB transmits the CSI-RS to the RN at a specific subframe, the eNB cannot always receive a signal from the RN at the specific subframe transmitting the CSI-RS. Likewise, if the RN transmits a CSI-RS to a relay UE at a specific subframe, the RN should always transmit a signal to the relay UE at the specific subframe, and cannot receive a signal from the eNB or the relay UE. This problem becomes a main cause of decreasing the flexibility of a system at RN operation.
Accordingly, there is a need for a more reasonable CSI-RS transmission/reception scheme in order to obtain a maximum gain through efficient RN support.